[MLIR] Add std.atomic_rmw op

Summary:
The RFC for this op is here: https://llvm.discourse.group/t/rfc-add-std-atomic-rmw-op/489

The std.atmomic_rmw op provides a way to support read-modify-write
sequences with data race freedom. It is intended to be used in the lowering
of an upcoming affine.atomic_rmw op which can be used for reductions.

A lowering to LLVM is provided with 2 paths:
- Simple patterns: llvm.atomicrmw
- Everything else: llvm.cmpxchg

Differential Revision: https://reviews.llvm.org/D74401